The invention relates to a light transmissive substrate carrying a light transmissive low ohmic coating and in particular to a cathode ray tube comprising a display screen carrying a light transmissive low ohmic coating.
The invention further relates to a method of manufacturing an low ohmic coating on a substrate.
Electroconductive coatings are inter alia used as antistatic layers on display screens of display devices, in particular cathode ray tubes (CRTs). Said layers have a sheet resistance, for example, of 106 to 1010 xcexa9/xe2x96xa1 and are hence sufficiently electroconductive to ensure that a high electrostatic voltage present on the outside surface of the display screen is removed within a few seconds. Thus, the user does not experience an unpleasant shock if he touches the screen. Besides, the attraction of atmospheric dust is reduced.
Since electromagnetic radiation may be hazardous to health, shielding from electromagnetic radiation is becoming ever more important. Cathode ray tubes, such as display tubes for TVs and monitor tubes, comprise a number of radiation sources which may be hazardous to the user""s health if he is exposed to said sources for a long period of time. A substantial part of the electromagnetic radiation generated can be screened off with metal in a simple manner via the housing of the cathode ray tube. However, radiation emitted via the display screen may substantially add to the amount of radiation to which the user is exposed.
This problem is solved by applying a well (electrically) conducting coating on the surface of the display screen. Said coating must also be sufficiently transparent in the wavelength range of from 400 to 700 nm, i.e. the transmission must be at least 60%. A well-known material which can be used for a transparent and well-conducting coating which meets said requirements is indium-doped tin oxide (ITO). Such a layer can be provided by means of vacuum evaporation or sputtering. Said method requires, however, expensive vacuum equipment. ITO layers can also be manufactured by firing spin-coated or sprayed layers of solutions of indium-tin salts. Said firing operation should be carried out at a temperature of at least 300xc2x0 C. This temperature is much too high to be used with a complete display tube which, in order to preclude damage to parts of the display tube, can withstand temperatures of maximally 160xc2x0 C.
In German Patent Application DE-A4229192, a description is given of the manufacture of an antistatic coating for, inter alia, a display screen, said coating being made from poly-3,4-ethylene dioxythiophene and a trialkoxysilane to improve the adhesion. By way of example, a coating is manufactured by providing a desalinated aqueous solution of poly-3,4-ethylene dioxythiophene, polystyrene sulphonic acid and 3-glycidoxypropyl trimethoxysilane on a glass plate, whereafter said glass plate is dried. Said poly-3,4-ethylene dioxythiophene is previously prepared by oxidatively polymerizing the monomer 3,4-ethylene dioxythiophene by means of an Fe(III) salt in water in the presence of polystyrene sulphonic acid to preclude precipitation. The antistatic layer thus obtained has a thickness of 0.6 xcexcm (600 nm) and a sheet resistance of 50 kxcexa9/xe2x96xa1. This sheet resistance is sufficient to bring about an antistatic effect.
A disadvantage of said known layer is that the shielding against electromagnetic radiation is insufficient. Future standards require the electrical field intensity measured at a distance of 0.3 m from the display screen to be maximally 10 V/m in the frequency range 50 Hz-2 kHz and 1 V/m in the frequency range 2-400 kHz. Experiments have shown that in order to meet these requirements the sheet resistance must be below 3 kxcexa9/xe2x96xa1 and preferably maximally 1 kxcexa9/xe2x96xa1, taking into account that the sheet resistance may increase with time.
A property of the known antistatic layer is that it is of a blue colour, although it is transparent. Since the sheet resistance is inversely proportional to the layer thickness, a greater layer thickness will lead to a lower sheet resistance. However, as a result thereof the transmission of the layer in the orange-red wavelength range decreases substantially and the blue colour becomes even more intense.
It is an object of the invention to provide, inter alia, a substrate, like a display screen of a cathode ray tube, carrying a coating, said coating providing an effective shield against electromagnetic radiation and exhibiting good optical properties, such as a transmission of at least 60% in the wavelength range of from 400 to 600 nm. Preferably, the layer must be compatible with additional antireflective layers. A further object of the invention is to provide a simple method of manufacturing such light transmissive well-conducting coatings, and it must be possible, in particular, to carry out said method at relatively low temperatures (maximally 160xc2x0 C.) at which no damage is caused to parts of a cathode ray tube.
These objects are achieved by a coated substrate as described in the opening paragraph, which is characterised according to the invention in that the coating is a mixed organic conductive polymer/transparent metal oxide coating having a layer thickness between 100 and 600 nm and a sheet resistance of less than 1 kxcexa9/xe2x96xa1. Depending on the thickness and/or the transparent metal oxide amount the sheet resistance can be between 100 and 600 xcexa9/xe2x96xa1. In accordance with the above-mentioned requirements, such a layer provides an excellent shield against electromagnetic fields. In addition, the composition of the coating is such that it can exhibit a transmission in excess of 60% in the wavelength range of from 400 to 700 nm. Metal oxides, like TiO2, and in particular SiO2 are suited for use in the mixed conductive polymer/transparent metal oxide coating.
The much lower sheet resistance of the coating in accordance with the invention as compared to the known coating is obtainable by the method of preparing the coating as described hereinbelow.
An electroconductive coating in accordance with the invention, optionally with one or more additional scratch resistant layers can also suitably be used as a touch screen coating on a CRT or LCD display screen. By touching a certain part of the touch screen coating on the display screen, a local change in resistance is induced which is translated, via electronic controls, into a localisation and a subsequent action, such as opening a menu, turning pages etc. It is alternatively possible to write on the display screen with a pen, whereafter the writing is identified and processed.
For the additional layer use can possibly be made of a silicon dioxide layer having a thickness of from 50 to 250 nm. Using a tetraalkoxysilane, such as TEOS, as the precursor, such a layer can be provided in a simple manner by means of a sol-gel process, followed by curing at a relatively low temperature (xe2x89xa6160xc2x0 C.).
The object of providing a simple method of manufacturing a transmissive electroconductive coating on a substrate (like a display screen of a cathode ray tube) is achieved in an embodiment in that the coating is manufactured by applying a layer of a solution of 3,4-ethylene dioxythiophene and an Fe(III) salt on the substrate, whereafter a treatment at an increased temperature is carried out, thereby forming a layer comprising poly-3,4-ethylene dioxythiophene and an Fe(III) salt, after which the layer is rinsed with an ethanolic metal oxide precursor, such as a SiO2 precursor (e.g. a tetra alkoxy silane like TEOS) which is capable to extract Fe salts, thereby forming the electroconductive coating. Optionally an organic base can be added, a.o. to stabilise the system.
In general, polymers are slightly soluble in solvents such as alcohols. In order to obtain a processable polymeric solution, in the known method the polymerisation reaction is carried out in the presence of a large quantity of a stabilising polymer, such as polystyrene sulphonic acid. Said polymer, however, leads to an increase of the sheet resistance. In the method in accordance with the invention, instead of a solution of the polymer, a solution of the monomer is provided on the surface of the display screen. The monomer 3,4-ethylene dioxythiophene is subsequently converted to the polymer. The monomer 3,4-ethylene dioxythiophene is converted to the corresponding polymer by means of oxidation with an Fe(III) salt. Fe(III) salts are very suitable because of the redox potential (Ered=0.77 V at room temperature) which is very favourable for this reaction. Fe(III) salts of organic sulphonates are very suitable because of their high solubility in alcohols and low crystallisation rate in the liquid layer to be provided. Examples of said salts are Fe(III)-p-toluene sulphonate and Fe(III)-ethylbenzene sulphonate.
Solutions of 3,4-ethylene dioxythiophene monomers and Fe(III) salt, which is necessary for the polymerisation reaction, are instable. When said components are mixed, a polymer soon forms in the solution, as a result of which the pot-life of the coating solution becomes impractically short. Surprisingly, it has been found that the reaction rate of the polymerisation reaction is decreased by adding small quantities of a soluble organic base to the coating solution. Dependent upon the concentration of the base, the reaction at room temperature can be suppressed completely. In the case of an efficacious base concentration, solutions comprising monomers and the Fe(III) salt can remain stable at room temperature for at least 24 hours: polymerisation is not apparent within this time period. These stable solutions can be used to apply thin layers to the display screen by, for example, spin coating. After heating of the layer, electroconductive poly-3,4-ethylene dioxythiophene is formed. Besides, it has been found that the addition of the organic base has a favourable effect on the conductivity of the polymer and hence on the sheet resistance of the conductive coating. Presumably, the organic base forms a complex with the Fe(III) salt, which results in a reduction of the redox potential at room temperature. This leads to a reduction of the reaction rate, so that a more controlled polymerization at an increased temperature takes place and the specific electrical conductivity increases by approximately a factor of two.
Suitable soluble bases for this method include, for example, imidazole, dicyclohexylamine and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU).
Said compounds can readily be dissolved in various alcohols, such as isopropanol and 1-butanol. A solution of said compounds, for example, in 1-butanol is used as the coating solution and has a pot-life of approximately 12 hours. Preferably, before the coating solution is used, it is filtered over an 0.5 xcexcm filter.
The coating solution can be provided on the substrate, like a CRT or LCD display screen, by means of customary methods, such as spraying or atomising. The solution is preferably spin coated onto the display screen. This results in a smooth, homogeneous and thin layer. During spin coating, the layer provided is dried and subsequently heated to a temperature of maximally 160xc2x0 C. by means of a furnace, a jet of hot air or an infrared lamp. At a temperature between 100 and 150xc2x0 C., the polymerisation reaction is completed within 2 minutes. The increased temperature initiates the polymerisation reaction in which the Fe(III) salt is converted to the corresponding Fe(II) salt. The colour of the coating changes from yellow to blueish green. The eventual thickness of the coating depends on the number of revolutions during spin coating and on the concentration of the dissolved compounds.
Removal of The Fe(III) and Fe(II) salts prevents the polymerised coating from becoming a dull layer as a result of crystallisation. In addition, the Fe(II) salt leads to an increase of the sheet resistance of the coating by a factor of ten. The Fe salts are removed by rinsing the coating with a suitable solvent. In this process, the Fe salts are extracted from the coating. Surprisingly, rinsing with an ethanolic SiO2 precursor like TEOS results in a mixed Polymer/SiO2 layer having attractive optical (anti-reflective) and electrical properties.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.